1. Field of the Invention
The invention relates generally to gain control amplifier circuits and, more particularly, to an improvement which reduces the dependency of the noise figure (NF) on the level of gain control.
2. Description of the Prior Art
Variable-gain amplifiers ("VGA's") are used in numerous electronic products such as global positioning (GPS) receivers, wireless local area networks and mobile communication devices, such as cordless and cellular phones. In particular, VGA's are used in various parts of such devices, for example, in the radio frequency (RF), intermediate frequency (IF) and the low frequency or baseband circuits, of these devices.
Traditionally, the gain of a RF VGA has been controlled 20 in discrete steps by using a current-steering (or current splitting) techniques. A typical current-steering VGA 100 is illustrated in FIG. 1. This circuit is known from the Journal: C. D. Hull, A Direct-Conversion Receiver for 900 MHZ (ISM Band) Spread-Spectrum Digital Cordless Telephone, 31 IEEE Journal of Solid-State Circuits, no. 12, pp. 1955-1963 (December 1996). Transistors Q1, Q2 and Q3 form a common-emitter transconductance stage 110 which converts radio-frequency RF input power RF.sub.in into current. The transconductance stage is typically, but not always, degenerated by impedance Ze to improve its linearity. In a high-gain mode of the VGA 100, the voltages at nodes B1 and B2 are set low to turn off the transistors Q6 and Q8. In this mode of operation, the transistors Q4, Q5 and Q7 function as cascode transistors of the transconductance stage. The resistor R1 functions as an output matching resistor. The inductor L1 and the capacitor C1 form an impedance-transformation network to transform the resistance of the resistor R1 to match that of an external load resistor (not shown in FIG. 1). The inductor L1 also serves as a pull-up inductor to increase the headroom at the collectors of the cascode transistors (Q4, Q5 and Q7).
In a low-gain mode, the transistors Q5 and Q7 are turned off and the voltages at nodes B1 and B2 are set high to render the transistors Q6 and Q8 conductive. This steers current away from the transistors Q5 and Q7 and dumps the output current from the transistors Q2 and Q3 to the power supply. As a result, the gain of the VGA is reduced, because less current is available at the output RF.sub.out for a given input signal RF.sub.in. In a medium-gain mode, the voltage at the node B1 is set low but the voltage at the node B2 is set high. The transistors Q5 and Q8 are conductive, but the transistors Q6 and Q7 are turned off. As a result, the transistors Q1 and Q2 of the transconductance stage supply output currents to node X, but output current from the transistor Q3 is dumped to the power supply.
In the current-steering gain-control scheme of FIG. 1, the gain steps between different gain modes depend on the device size ratios among the transistors Q1, Q2 and Q3. For instance, if the device size ratios Q1:Q2:Q3 are 1:1:2, the gain differs by a factor of 2 between the each successive gain mode, providing a uniform gain step of 6 dB.
A disadvantage of the known VGA shown in FIG. 1 is that it has higher noise figures in the low-gain and medium-gain modes than in the high-gain modes. The noise figure (NF) of a circuit measures the degradation of signal-to-noise ratio (SNR) caused by the circuit. Signal-to-noise ratio is defined as: SNR in decibels=10 log(signal power/noise power). The noise figure is defined as: NF in decibels=(SNR at input in dB)-(SNR at output in dB). Throwing away some of the signal current by dumping to power supply, as in the low and medium gain modes of the LNA of FIG. 1, decreases the signal power, and hence degrades the noise figure by reducing the SNR at the output of the LNA. The current-steering gain-control approach of FIG. 1 simply causes too high of a NF in the low-gain modes to be useful in certain applications.
Accordingly, it is an object of the invention to provide a gain-control amplifier circuit with reduced NF degradation across multiple gain modes.